1. Field of the Invention
The objective of the invention appears from the title. Obliquely oriented film is mainly used for manufacture of crosslaminates, whereby two or more such films are continuously brought together in a way to make their directions of orientation cross each other. The bonding may take place by a tie film extruded at the location where the oriented films meet (i.e. “extrusion lamination) or by heat and pressure through coextruded surface layers. The technology of producing crosslaminates is extensively described in WO 08/006858 (Rasmussen) published by WIPO. The polymer compositions of film, which have been industrially used in crosslaminates, have mainly been based on HDPE, LLDPE (and blends of the two) or crystalline PP.
2. Description of the Related Art
Almost all industrially used crosslamination technology makes use of helical cutting of oriented tubular film. The first patent to this effect is GB 816,607 (Rasmussen) which claims priority from 1954. A particularly practical way of carrying out such process and apparatus for such process are known from U.S. Pat. No. 5,248,366 (Rasmussen) and U.S. Pat. No. 5,361,469 (Rasmussen) both claiming priority from 1988.
The manufacture of an obliquely oriented film for crosslamination requires stabilization of the orientation prior to the lamination, otherwise the different films in the laminate will gradually build up internal tensions when stored on reel, the tensions having different directions in the different films corresponding to their different directions of orientation, and this will give the laminate a strong tendency to curl or roll up, when it is released from the reel. It is known that, with conventional technology, at least a part of this stabilization must take place before the oriented lay-flat tube is wound up, otherwise the film on the reel will become distorted, and the core of the reel may even be crushed.
As explained in WO 05/102669 (Rasmussen), the stretching is preferably carried out at a relatively low temperature, which for HDPE or PP films preferably is in a range of about 20-50° C., since this gives the best all-round strength properties. The latter refers to an adequate combination of tensile strength, yield point, tear propagation resistance and puncture resistance. However, the choice of low temperatures makes the choice of stabilization conditions more difficult, too low a stabilization giving the final crosslaminate a high tendency to curl, and too high a stabilization making the film too splittable, thereby reducing the tear propagation resistance of the crosslaminate. A part of the problem is that, under industrial conditions with relatively high film velocities, there has to be a relatively high tension in the machine direction of the film, i.e. parallel with its direction of orientation.